Backlight using planar hologram for flat display device

ABSTRACT

The backlight for a flat display device including a light source, a light guide plate and a surface hologram is provided. The light guide plate is installed at one side of the light source, and light from the light source travels in the light guide plate while being totally reflected. The surface hologram is formed on at least one surface of the light guide plate. The surface hologram has a pattern of a predetermined grating interval and a predetermined grating depth in order to diffract light at a predetermined angle toward the light guide plate.

BACKGROUND OF THE INVENTION

[0001] This application claims the priorities of Korean PatentApplication Nos. 2001-38807 and 2002-23114, filed on Jun. 30, 2001 andApr. 26, 2002, respectively, in the Korean Intellectual Property Office,the disclosures of which are incorporated herein in their entireties byreference.

[0002] 1. Field of the Invention

[0003] The present invention relates to the field of backlights for flatdisplay devices, and more particularly, to a backlight for a flatdisplay device including a light path changing unit, which makes alreadyincident light again incident upon a planar hologram at an angleproviding the maximum diffraction efficiency, and a light guide plateincluding the planar hologram.

[0004] 2. Description of the Related Art

[0005] Referring to FIG. 1, a conventional backlight includes a lightsource 11, a light guide plate 13, a diffusion plate 19 and prismaticplates 21 a and 21 b. The light guide plate 13 directs light emittedfrom the light source 11, the diffusion plate 19 diffuses light directedby the light guide plate 13, and the prismatic plates 21 a and 21 bcollect diffused light on their respective front sides.

[0006] Referring to FIG. 2, in a conventional backlight, light emittedfrom the light source 11 is incident upon the light guide plate 13 andfurther transmitted by total reflection. Some of the light passesthrough the bottom surface of the light guide plate 13, is reflected bya reflection plate 17 and reenters the light guide plate 13.

[0007] A light scattering pattern 23, which is a dot pattern, is printedon the bottom surface of the light guide plate 13. From light incidentupon the light guide plate 13, only light not reflected due toscattering by the light scattering pattern 23 penetrates through theupper surface of the light guide plate 13.

[0008] To improve light uniformity, the diffusion plate 9, which coversthe entire surface of the light guide plate 13, diffuses the lightemitted from the light guide plate 13.

[0009] The prismatic plates 21 a and 21 b are provided on the entiresurface of the diffusion plate 19. Each of the prismatic plates 21 a and21 b is formed of a plurality of prism strips with triangular crosssections. When two prismatic plates are used together, as in the case ofthe prismatic plates 21 a and 21 b, they are disposed so that theprismatic strips of the prismatic plate 21 a make a right angle with theprismatic strips of the prismatic plate 21 b. The prismatic plates 21 aand 21 b increase the front luminance of light by refracting or totallyreflecting light emitted from the diffusion plate 19 depending on theangle at which light is incident upon the prismatic plates 21 a and 21b.

[0010] A protection plate 25 is installed on the front side of theprismatic plate 21 a to protect the entire system of backlight.

[0011] In such a conventional backlight, in order to emit light from thelight guide plate 13, a dot pattern for light scattering is formed onthe bottom surface of the light guide plate 13, or the light guide plate13 has a prismatic or sinusoidal bottom surface. However, manufacturingof a prismatic or sinusoidal pattern on the light guide plate 13requires a special and expensive equipment and a time-consuming process.

[0012] The dot pattern printed on the bottom surface of the light guideplate 13 makes spots appear on the screen when it is combined with aliquid crystal panel, thus degrading image quality. A diffusion plateadopted to solve this problem reduces the light reflection efficiency by20% to 30% depending on the transmission performance. As the lightefficiency of the light guide plate 13 depends on the position and areaof its dot pattern, it may be further reduced according to the positionand area of the dot pattern.

[0013] In a conventional backlight, light emitted from the light source11 is scattered by a dot pattern for light scattering or prism-shapedgrooves formed on the bottom surface of the light guide plate 13, andthen emitted at an angle of 70° to 90° with respect to a normal lineperpendicular to the light guide plate 13. A diffusion plate and aprismatic plate are further required to convert the direction of suchlight emitted at a large angle into the normal direction of the lightguide plate 13. As a result, the assembling process of a backlight iscomplicated, and the manufacturing costs thereof increase.

[0014]FIGS. 3A through 3E are graphs showing a variation in thedistribution of light intensity for a conventional backlight. FIG. 3Ashows the light intensity distribution of light emitted from the lightguide plate 13 of FIG. 2. Referring to FIG. 3A, the light intensitydistribution of light emitted from the light guide plate 13 has anasymmetrical structure where the highest light intensity is obtainedaround 90°. That is, the light intensity around 90° is about 3,400 cd(where cd denotes the base unit of light intensity, i.e., candela), anda light intensity of 400 cd or less appears between 0° and 180°.

[0015] The asymmetrical light intensity distribution of FIG. 3A changesinto a symmetrical light intensity distribution in which the lightintensity of the center portion of a conventional backlight becomesstronger, as shown in FIGS. 3B to 3E.

[0016]FIG. 3B shows the light intensity distribution of light emittedfrom the light guide plate 13 and the diffusion plate 19. Referring toFIG. 3B, an asymmetrical distribution is still shown where a portionwith the highest light intensity of about 800 cd appears on the upperhalf vertical axis and the light intensity becomes weaker from thecenter of the conventional backlight toward the periphery thereof.

[0017]FIG. 3C shows the light intensity distribution of lighttransmitted by the light guide plate 13, the diffusion plate 19 and thefirst prismatic plate 21 b. Referring to FIG. 3C, the highest lightintensity of about 940 cd appears around the center of the conventionalbacklight. The light intensity distribution of FIG. 3C is symmetrical incontrast with the light intensity distribution of FIG. 3B.

[0018]FIG. 3D shows the light intensity distribution of lighttransmitted by the light guide plate 13, the diffusion plate 19 and thefirst and second prismatic plates 21 b and 21 a. The light intensitydistribution of FIG. 3D has a similar shape to that of FIG. 3C rotatedby 90°. The light intensity at the center of the conventional backlightis about 1,220 cd, and the distribution of light intensity issymmetrical.

[0019]FIG. 3E shows the distribution of the final light intensity of theconventional backlight. Referring to FIG. 3E, the light intensity at thecenter of the conventional backlight is about 1,100 cd, and thedistribution of the final light intensity is symmetrical.

[0020] Conventional backlights must include a prismatic plate in orderto obtain such a symmetrical light intensity distribution as shown inFIG. 3E. This leads to complicated, expensive backlights.

[0021] As described above, in the prior art, an additional device isrequired to compensate for the large emission angle of light emittedfrom a light guide plate. This causes an increase in the manufacturingcosts of backlights.

SUMMARY OF THE INVENTION

[0022] To solve the above-described problems, it is an object of thepresent invention to provide a backlight for a flat display devicecapable of reducing light loss, which is generated while light ispassing through a diffusion plate and a prismatic plate so that light isoutput from a light guide plate at an angle of 90° or at a predeterminedangle near 90° with respect to the plane of the light guide plate, or toprovide a no plastic plate backlight for a flat display device.

[0023] Another object of the present invention is to provide a backlightfor a flat display device capable of using specific polarized lightdepending on the specific wavelength of light emitted from a lightsource.

[0024] Still another object of the present invention is to provide abacklight for a flat display device capable of compensating for theinverse proportional relationship between the diffraction efficiency andlight intensity of a light guide plate on which a diffraction gratingpattern is formed, both the diffraction efficiency and the lightintensity depending on the incidence angle.

[0025] In order to achieve the above objects of the present invention,there is provided a backlight for a flat display device including alight source, a light guide plate and a surface hologram. The lightguide plate is installed at one side of the light source. In the lightguide plate, light from the light source travels while being totallyreflected. The surface hologram is formed on at least one surface of thelight guide plate. The surface hologram diffracts and emits the light ata predetermined angle to the plane of the light guide plate.

[0026] Preferably, the backlight for a flat display device furtherincludes a light path changing unit installed at one side of the lightguide plate. The light path changing unit changes the path of lighttraveling in the light guide plate to make light incident upon theplanar hologram at an angle near the angle providing the maximumdiffraction efficiency.

[0027] The backlight for a flat display device can further include areflecting plate installed on the rear side of the light guide plate.The reflecting plate reflects light diffracted by the planar hologramand sends the diffracted light back to the light guide plate.

[0028] The light path changing unit is a reflective mirror that islocated opposite to the light source and inclined at a predeterminedangle. Alternatively, the light path changing unit is a reflectivesurface of the light guide plate, the reflective surface being locatedopposite to the light source and inclined at a predetermined angle.Still alternatively, the light path changing unit is a refractingelement installed between the light source and the light guide plate orinstalled opposite to the side of the light guide plate where the lightsource is installed.

[0029] Here, the refracting element is either a refractive lens or arefractive grating.

[0030] Preferably, the diffraction efficiency of the planar hologrambecomes lower toward either the light source or the light path changingunit.

[0031] Preferably, the pattern of the planar hologram becomes smallertoward either the light source or the light path changing unit.

[0032] It is preferable that the grating interval of the planar hologramis 2 μm or less.

[0033] In order to diffract incident beams into optimal states, thegrating interval of the planar hologram can be composed of at least twotypes of grating intervals depending on the wavelength of light.

[0034] As for the surface hologram, the grating depth can be set so thatthe polarization of light output from the light guide plate after beingdiffracted by the surface hologram is superior to the direction ofspecific polarized light. Accordingly, it is preferable that the gratingdepth of the surface hologram is set so that the diffraction efficiencyof a specific polarized light beam is 1.5 times as large as or greaterthan that of the other polarized light that meets the specific polarizedlight beam at a right angle.

[0035] Preferably, the backlight for a flat display device furtherincludes a diffusion plate installed on the entire surface of the lightguide plate to diffuse light emitted from the light guide plate.

[0036] When a white light source is used, the diffusion can reduce thedegree of color separation of light caused by the difference indiffraction angle between light wavelengths.

[0037] In the present invention, a light path changing unit can increasethe output amount of light by changing the path of high intensity lightincident at an angle near 90°, so that the incident light is againincident upon the surface hologram at an angle guaranteeing the maximumdiffraction efficiency.

[0038] Also, the planar hologram formed on the light guide plate enableslight incident upon the light guide plate to be output from the lightguide plate at an angle nearly perpendicular to the light guide plate.Accordingly, an efficient backlight having a simple structure having noprismatic plates can be provided.

[0039] In addition, the diffraction efficiency of a specific polarizedlight beam is rendered greatly higher than that of other polarized lightbeams by controlling the grating interval and depth of the surfacehologram. Thus, the specific polarized light beam of light emitted fromthe light guide plate becomes excellent. Finally, the backlightaccording to the present invention can increase the light efficiencyusing polarization. As a result, a backlight using specific polarizationcan be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The above and other features and advantages of the presentinvention will become more apparent by describing in detail exemplaryembodiments thereof with reference to the attached drawings in which:

[0041]FIG. 1 is a schematic perspective view of a conventionalbacklight;

[0042]FIG. 2 is a cross-section of a conventional backlight;

[0043]FIG. 3A shows the light intensity distribution of lighttransmitted by a light guide plate in a conventional backlight;

[0044]FIG. 3B shows the light intensity distribution of lighttransmitted by a diffusion plate in a conventional backlight;

[0045]FIG. 3C shows the light intensity distribution of lighttransmitted by a first prismatic plate in a conventional backlight;

[0046]FIG. 3D shows the light intensity distribution of lighttransmitted by a second prismatic plate in a conventional backlight;

[0047]FIG. 3E shows the light intensity distribution of final light in aconventional backlight;

[0048]FIG. 4A is a schematic cross-section of a backlight according to afirst embodiment of the present invention;

[0049]FIG. 4B is a schematic cross-section of a backlight according to asecond embodiment of the present invention;

[0050]FIG. 5A is a schematic cross-section of a backlight according to athird embodiment of the present invention;

[0051]FIG. 5B is a schematic cross-section of a backlight according to afourth embodiment of the present invention;

[0052]FIG. 6A is a schematic cross-section of a backlight according to afifth embodiment of the present invention;

[0053]FIG. 6B is a schematic cross-section of a backlight according to asixth embodiment of the present invention;

[0054]FIG. 7 is a schematic cross-section of a flat backlight having nolight path changing units, according to an embodiment of the presentinvention;

[0055]FIG. 8 is an enlarged cross-section of a portion indicated byreference character A of FIG. 7;

[0056]FIG. 9 is a graph showing the diffraction efficiency, lightintensity distribution and output light quantity with respect toincidence angles of the flat backlight of FIG. 7;

[0057]FIG. 10A shows a stripe-patterned planar hologram formed on alight guide plate in the backlight of FIG. 7;

[0058]FIG. 10B shows a stripe-patterned planar hologram formed on alight guide plate in the backlight of FIG. 4A;

[0059]FIG. 11A shows a rectangle-patterned planar hologram formed on alight guide plate in the backlight of FIG. 7;

[0060]FIG. 11 B shows a rectangle-patterned planar hologram formed on alight guide plate in the backlight of FIG. 4A;

[0061]FIG. 12A shows a stripe-patterned planar hologram formed on alight guide plate in the backlight of FIG. 7;

[0062]FIG. 12B shows a stripe-patterned planar hologram formed on alight guide plate in the backlight of FIG. 4A;

[0063]FIG. 13 schematically shows a method of forming a flat hologramused by a backlight for a flat display device according to the presentinvention;

[0064]FIG. 14 is a graph showing the diffraction efficiency ofP-polarized light and S-polarized light according to the depths of thegrating of a planar hologram when light with a 460 nm wavelength andlight with a 620 nm wavelength are incident upon a planar hologram witha grating interval of 440 nm;

[0065]FIG. 15A shows the light intensity distribution of lighttransmitted by a light guide plate in the backlight of FIG. 7; and

[0066]FIG. 15B shows the light intensity distribution of final lighttransmitted by a diffusion plate in the backlight of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

[0067]FIGS. 4A through 6B show backlights for a flat display deviceaccording to the first through sixth embodiments of the presentinvention, respectively, the backlights having a light path changingunit. In FIGS. 4A and 4B, the light path changing unit is a slantreflection plane formed on the side of a light guide plate.

[0068] Referring to FIG. 4A, a backlight for a flat display deviceaccording to a first embodiment of the present invention includes alight source 61, a light guide plate 63, a reflection plane 69 a, aplanar hologram 65 and a reflection plate 67. The light source 61 emitslight l1. The light guide plate 63 directs the light l1 emitted from thelight source 61 by totally reflecting the light l1. The reflection plane69 a is used as a light path changing unit for changing the path of thelight l1. The planar hologram 65 diffracts light to output light at anangle of no less than 80° or at a predetermined design angle withrespect to the plane of the light guide plate 63. The reflection plate67 is positioned at the bottom surface of the planar hologram 65,reflects light diffracted by the planar hologram 65, and sends reflectedlight back to the light guide plate 63.

[0069] The light source 61 can be one of a laser diode (LD), a lightemitting device (LED) and a cold cathode fluorescent lamp (CCFL). Asdescribed above, the planar hologram 65 is used in the presentinvention.

[0070] Generally, holograms can be classified into surface holograms andvolume holograms. In volume holograms, an image is recorded within ahologram material. In surface holograms, an image is recorded on itssurface. Consequently, a surface hologram can be mass-produced throughduplication, but usually provides low diffraction efficiency. However,when a hologram with a grating interval having a similar length to thewavelength of light is formed on a light guide plate, light is notscattered in many directions due to the small number of diffractionorders capable of being transmitted. This case can overcome lowdiffraction efficiency.

[0071] A backlight for a flat display device according to the presentinvention reduces the, dependency upon the diffraction angle ofreproduced light using a planar hologram and also compensates for thedefects of the planar hologram by increasing the diffraction efficiencyby introducing a light path changing unit.

[0072] In the backlight for a flat display device of FIG. 4A, the lightl1 emitted from the light source 61 is incident upon the light guideplate 63 at an angle where total reflection can be achieved with respectto the plane of the light guide plate, and travels through totalreflections. Here, light intensity increases as an initial incidenceangle A approaches 90°, and the diffraction efficiency with respect tothe planar hologram 65 is highest at an incidence angle C rangingbetween 40° and 60°. Accordingly, the initial incidence angle A of lightis set at an angle between the total reflection angle Θ and 90°. Theincidence angle C is set to an angle where the refraction efficiencywith respect to the planar hologram 65 is the highest, by controllingthe slant angle of the reflection plane 69 a.

[0073] The total reflection angle can be obtained through the Snell'slaw. Since the refractive index (n) of the light guide plate 63 isusually about 1.5, the total reflection angle Θ for totally reflectingthe light l1 within the light guide plate 63 can be obtained fromExpression 1: $\begin{matrix}{{a\quad \sin \quad ( \frac{1}{n} )} - {a\quad \sin \quad ( \frac{1}{1.5} )} - {41.8{^\circ}}} & (1)\end{matrix}$

[0074] When the incidence angle A of the light l1 ranges from the totalreflection angle Θ to 90° and the angle for the highest diffractionefficiency is 40°, the incidence angle C with respect to the planarhologram 65 is obtained from Equation 2:

C=A−2B   (2)

[0075] The inclination B of the reflection plane 69 a ranges from 0.9°to 25° according to Expression 3: $\begin{matrix}{{0.9 - \frac{41.8 - 40}{2}} < {B - \frac{A - C}{2}} < {\frac{90 - 40}{2} - 25}} & (3)\end{matrix}$

[0076] Since the light amount increases as the incidence angle Aapproaches 90°, in order to maximally diffract light having an incidenceangle A close to 90°, for example, an 85° incidence angle A, a 17.5°inclination angle B for a reflection plane can be obtained bysubstituting 85° for the initial incidence angle A in Equation 2 and 50°for the incidence angle C in Equation 2. The incidence angle of lightupon the planar hologram 65, which provides the maximum diffractionefficiency, can be properly controlled according to the pattern of theplanar hologram 65 because it depends on grating depths and gratingintervals.

[0077] If light is incident upon the planar hologram 65 at an incidenceangle C close to the incidence angle providing the maximum diffractionefficiency, light diffracted by the planar hologram 65 is reflected bythe reflection plate 67 and penetrates through the planar hologram 65.Some of the light transmitted by the planar hologram 65 re-enters thelight guide plate 63, and the rest is output through the light guideplate 63. Here, the output angle of light is set to an angle of 80° ormore by controlling the grating interval and the grating depth. Thiswill be described in detail when a backlight according anotherembodiment of the present invention having no light path changing unitsis described. In some cases, the angle at which light is output can becontrolled by adjusting the grating interval.

[0078]FIG. 4B shows a backlight according to a second embodiment of thepresent invention. Unlike the backlight for a flat display deviceaccording to the first embodiment of the present invention of FIG. 4A,the planar hologram 65 is formed on the upper side of the light guideplate 63. In addition, a reflection plane 69 b is inclined in theopposite direction to the reflection plane 69 a of FIG. 4A, so that theincidence angle C with respect to the plane of the planar hologram 65approaches the incidence angle providing the maximum diffractionefficiency. However, as long as the incidence angle C satisfies thetotal reflection angle, the reflection plane 69 b may not be inclined inthe opposite direction to the reflection plane 69 a of FIG. 4A. If ahologram pattern is formed on the upper surface of a light guide plate,like in this embodiment, a reflection sheet 67 may not be included.

[0079] As described above, a maximum output light amount is extracted bymaking light with high light intensity incident upon the planar hologram65, which is formed on the upper surface of the light guide plate 63, atan angle close to the angle providing the maximum diffractionefficiency.

[0080] Backlights of FIGS. 5A and 5B adopt reflection mirrors 68 a and68 b, respectively, instead of the reflection planes 69 a and 69 b ofthe backlights according to the first and second embodiments of thepresent invention. FIG. 5A shows a backlight for a flat display deviceaccording to a third embodiment of the present invention.

[0081] Referring to FIG. 5A, light l3 is incident upon the upper side ofthe light guide plate 63 at an incidence angle A. The incident light l3passes through the light guide plate 63 and is then reflected by thereflection mirror 68 a. The reflected light l3 re-enters into the lightguide plate 63 and is then incident upon the planar hologram 65 at anincidence angle C. As described above, when the incidence angle A isclose to 90°, the light intensity of the light l3 is the highest. Thelight l3 is incident upon the planar hologram 65 at the incidence angleC close to the angle providing the maximum diffraction efficiency.

[0082]FIG. 5B shows a backlight according to a fourth embodiment of thepresent invention. Referring to FIG. 5B, the light guide plate 63 hasthe planar hologram 65 formed on its upper side. In order to change theincidence angle of light l4 upon the planar hologram 65, the inclinationangle B of the reflection mirror 68 b is set in the same way as theinclination angle B of the reflection mirror 68 a for the backlight ofFIG. 5A.

[0083]FIG. 6A shows a backlight according to a fifth embodiment of thepresent invention. Referring to FIG. 6A, a diffraction grating 66 a isincluded as a light path changing unit between the light source 61 andthe light guide plate 63. The diffraction grating 66 a changes the pathof light l5 incident upon the plane of the light guide plate 63 at anangle close to 90°, so that the light l5 travels to be incident upon theplanar hologram 65 at an angle close to the incidence angle providingthe maximum diffraction efficiency. The incidence angle C with respectto the planar hologram 65 has a range as described above, and the anglefor the maximum diffraction efficiency may vary within the range of theincidence angle C, depending on grating depths and grating intervals.Here, the diffraction grating 66 a can be replaced by a refraction lens.

[0084]FIG. 6B schematically shows a backlight for a flat display deviceaccording to a sixth embodiment of the present invention. Referring toFIG. 6B, a refraction lens 66 b is located opposite the light source 61,and has a reflective right side. Light l6 is reflected by the reflectiveright side and sent back to the light guide plate 63. The reflectedlight is incident upon the planar hologram 65 at an angle C close to theangle for the maximum diffraction efficiency. The range of the angle Cis the same as described above.

[0085] The backlights for a flat display device according to the firstthrough sixth embodiments of the present invention are just examples ofthe present invention. Other types of light path changing units can beused if they can change the path of light and make light incident upon aplanar hologram at an angle close to the incidence angle for the maximumdiffraction efficiency. The new light path changing units used can beinstalled between the light source 61 and the light guide plate 63, bybeing formed on one side of the light guide plate 63, or by beinginstalled opposite the light source 61.

[0086] In addition, in the backlights for a flat display deviceaccording to the first through sixth embodiments of the presentinvention, a planar hologram 65 with a pattern to be described later isformed on one side of the light guide plate 63. That is, a planarhologram 65 with a pattern providing low diffraction efficiency or asmall pattern is formed at the position having a high light intensity,so that the output amount of polarized light can be maximized and theoutput light can be uniformly distributed. The output light can befurther uniformly distributed by adding a diffusion plate over the lightguide plate 63.

[0087]FIG. 7 schematically shows a flat backlight with no light pathchanging units, according to the present invention. The flat backlightof FIG. 7 includes a light guide plate 43, a light source 41, areflecting plate 47, and a planar hologram 45. The light source 41 isinstalled on one side of the light guide plate 43, the reflecting plate47 is installed under the light guide plate 43, and the planar hologram45 is formed on the bottom surface of the light guide plate 43.

[0088] A diffusion plate 49 is further installed over the light guideplate 43. A protection plate (not shown) can be further installed overthe diffusion plate 49. The planar hologram 45 can be also formed on theupper surface of the light guide plate 43.

[0089] Light 31, white light emitted from the light source 41 to theplastic light guide plate 43, travels while being totally reflectedwithin the light guide plate 43. The light 31 continuously remainswithin the light guide plate 43 as long as it satisfies the totalreflection conditions. The light 31 is diffracted by the planar hologram45 formed on the bottom surface of the light guide plate 43. First-orderlight beams 33, 35 and 37, from the diffracted light beams, arereflected by the reflecting plate 47 and sent back to the light guideplate 43. The reflected first-order light beams 33, 35 and 37 are outputfrom the light guide plate 43 at angles nearly perpendicular to theplane of the plastic guide 43, that is, at angles of 80° or more.Meanwhile, a zero order light beam 39 is again reflected by the planarhologram 45 because it satisfies the total reflection conditions.

[0090] Most light is incident at an angle of 80° or more because thelight guide plate 43 is long and narrow. Hence, when green light with awavelength (λ) of 540 nm is incident at an angle (θ) of 80°, first orderdiffracted light is emitted vertically by satisfying Expression 4:$\begin{matrix}{P - \frac{\lambda}{n \times \sin \quad \theta} - \frac{540}{1.5 \times \sin \quad 80{^\circ}} - {365\quad {nm}}} & (4)\end{matrix}$

[0091] wherein P denotes the grating interval, which is formed on aplanar hologram.

[0092] As shown in Equation 4, when green light is incident at 80° andthe grating interval P of the planar hologram 45 is about 365 nm, it isemitted nearly perpendicularly to the light guide plate. When light isincident at a different angle than 80° or light having a differentwavelength than green light is incident, the grating interval P of theplanar hologram 45 is changed to vertically emit the incident light. Inall of these cases, when the grating interval of the planar hologram 45is about 2 μm or less, light is uniformly emitted from the light guideplate 43 at a certain angle.

[0093] In the backlight of FIG. 7, light emitted from the light guideplate 43 makes an angle of almost 90° to the plane of the light guideplate 43, that is, an angle of 80° or more. Accordingly, the backlightof FIG. 7 does not require a prismatic plate, which is used inconventional backlights to obtain a high luminous efficiency. Thissimplifies the structure of a backlight, so that the backlight can beeasily manufactured. Even when there is no need to emit light at anangle of almost 90°, light can be emitted at a certain angle bycontrolling the grating interval.

[0094]FIG. 8 is an enlarged cross-section of a portion indicated byreference character A of FIG. 7, showing the path of travel of incidentlight 31 diffracted by the planar hologram 45. Referring to FIG. 8, zeroorder light 39 from the incident light 31 is reflected by the interfaceat the hologram 45 formed on the bottom surface of the reflecting plate47, while first-order diffracted light beams 33, 35 and 37 arediffracted by the hologram 45. That is, the first-order diffracted lightbeam 35, which is green, is diffracted at an angle of almost 90° to theplane of the light guide plate 43, while the first order light beams 33and 37, which are red and blue, respectively, are diffracted at an anglelarger than the diffraction angle of the green light beam 35.First-order diffracted light beams 33, 35 and 37 are reflected by thereflection plate 47 and sent back to the light guide plate 43 at anangle smaller than the total reflection angle. The first-orderdiffracted light beams 33, 35, and 37 not satisfying the totalreflection conditions get out of the light guide plate 43 and travels atan angle of 80° or more to the plane of the light guide plate 43.

[0095]FIG. 9 is a graph showing the diffraction efficiency f1, lightintensity distribution f2, and output light quantity f3 with respect toincidence angles when the light guide plate of the flat backlight ofFIG. 7 has a dot-patterned or rugged bottom surface with a gratinginterval of 440 nm and a grating depth of 0.25 nm. As shown in FIG. 9,the diffraction efficiency f1 remarkably decreases after the incidenceangle reaches 50°, while the light intensity distribution f2 remarkablyincreases after the incidence angle reaches 50°. The output lightquantity f3 of light output from a light guide plate corresponds to theproduct of the values of the two graphs f1 and f2, and has a maximumbetween 55° and 70°.

[0096] That is, in the backlight with no light path changing units, thediffraction efficiency of light is the lowest at the incidence angle of90° where the light intensity is the highest, such that the output lightquantity is generally lowered. However, the flat backlights having alight path changing unit, according to the first through sixthembodiments of the present invention, can reduce the loss of lightintensity and increase the diffraction efficiency.

[0097]FIG. 10A shows the pattern of a planar hologram 45 a formed on alight guide plate 43 a in a backlight with no light path changing unitsas described in FIG. 7. Referring to FIG. 10A, the planar hologram 45 ahas a stripe pattern. The stripes constituting the planar hologram 45 aare consecutively arranged on the entire bottom surface of the lightguide plate 43 a.

[0098] In a flat backlight according to the present invention, when thediffraction efficiency of the planar hologram 45 a is uniform over thelight guide plate 43 a, the planar hologram 45 a becomes smaller as itapproaches the light source 41 a, and it becomes bigger as it becomesmore distant from the light source 41 a. Alternatively, the pattern ofthe planar hologram 45 a becomes sparser as it approaches the lightsource 41 a, and it becomes denser as it distances from the light source41 a.

[0099] If the light source 41 a is located on the lateral side of thelight guide plate 43 a, the light intensity becomes higher as itapproaches the light source 41 a, and it becomes lower as it distancesfrom the light source 41 a.

[0100] Accordingly, in order to emit uniform light from the entiresurface of the light guide plate 43 a, it is preferable that planarholograms 45 a with low diffraction efficiency are arranged in thedirection approaching the light source 41 a and planar holograms 45 awith high diffraction efficiency are arranged in the directiondistancing from the light source 41 a. If the planar hologram 45 a hasuniform diffraction efficiency over the light guide plate 43 a, it ispreferable that the pattern of the planar holograms 45 a becomes smalleras it approaches the light source 41 a and becomes bigger as itdistances from the light source 41 a.

[0101] The striped-patterned planar hologram 45 a can partially beformed on the light guide plate 43 a. Preferably, the stripes of thedisplay hologram 45 a are periodically arranged. If the stripes of thedisplay hologram 45 a are not periodically arranged, light is notuniformly emitted from the planar hologram 45 a, thus causing adegradation in the light efficiency of the entire backlight.

[0102]FIG. 10B shows the pattern f a planar hologram 65 a formed on alight guide plate 63 a in the flat backlights according to the firstthrough fourth and sixth embodiments of the present invention. Referringto FIG. 10B, the pattern of the planar hologram 45 a becomes sparser inthe directions toward the light source 61 and the light path changingunit 69, and becomes denser in the direction toward the center of thelight guide plate 63 a.

[0103] As described above, the light intensity of the planar hologram 65a, near the light source 61 and the light path changing unit 69, isrelatively higher than that of the planar hologram 65 a at the center ofthe light guide plate 63 a. Accordingly, in order to uniformlydistribute the light intensity of light emitted from the light guideplate 63 a over the entire surface of the light guide plate 63 a, thediffraction efficiency of the planar hologram 65 a is reduced in thedirections toward the light source 61 and the light path changing unit69, and it is increased in the direction toward the center of the lightguide plate 63 a.

[0104] Here, the pattern of the planar hologram 65 a of FIG. 10B can beapplied to the flat backlights according to the first through fourthembodiments and the sixth embodiment of the present invention. Thegrating interval and the grating depth, depending on the incidence angleproviding the maximum diffraction efficiency, control the diffractionefficiency within a predetermined range. When a pattern having uniformdiffraction efficiency is formed, a planar hologram with a small patternis formed near the light source 6 and the light path changing unit 69,and a planar hologram with a large pattern is formed far from the lightsource 6 and the light path changing unit 69.

[0105]FIG. 11A shows a rectangle-patterned planar hologram 45 b formedon a light guide plate 43 b in the flat backlight with no light pathchanging units of FIG. 7. Referring to FIG. 11A, a planar holograms 45 bformed on a light guide plate 43 b is patterned with two types ofrectangles having different grating intervals. As described above, thegrating interval varies according to the wavelength of light.

[0106] The patterned rectangles of the planar hologram 45 b becomesmaller in the direction toward the light source 41 b, and they becomelarger in the direction distancing from the light source 41 b.

[0107] When the diffracting direction varies according to the wavelengthof light, the light intensity distribution depending on the angle variesaccording to the wavelength of light. This may cause a degradation inthe quality of image. Accordingly, the impression of a color shown on aliquid crystal panel may vary. If the planar hologram 45 b having theabove-described grating interval is formed on the light guide plate 43 bto make red light have the highest diffraction efficiency, thereproduced light may provide a stronger impression of red, which lowersthe image quality.

[0108] Accordingly, the planar hologram 45 b having at least two typesof grating intervals can be provided in order to balance the impressionof a color.

[0109] As described above referring to FIG. 11A, the planar hologram 45b can be patterned in one type of rectangles such that the red light isemitted forward, and in the other type of rectangles such that the bluelight is emitted forward.

[0110] The grating interval satisfying Equation 2 is about 528 nm forred light (λ=800 nm), and about 474 nm for blue light (λ=405 nm). Thus,the use of the planar hologram 45 b patterned with rectangles having thetwo types of grating intervals, i.e., 474 nm and 274 nm, can uniformlymaintain the color impression of emitted light.

[0111] The pattern for the planar hologram 65 b of FIG. 11B is obtainedby applying the pattern for the planar hologram 45 b of FIG. 11A, andcan be applied to the first through fourth and sixth embodiments of thepresent invention.

[0112] In FIG. 11B, the planar hologram 65 b is patterned withrectangles having two types of grating intervals based on thewavelengths of light as in FIG. 11A. However, the planar hologram 65 bis patterned so that the diffraction efficiency becomes lower in thedirections from the center of the light guide plate 63 b to both thelight source 61 and the light path changing unit 69. Alternatively, whenthe diffraction efficiency is uniform over the entire surface of thelight guide plate 63 b, the planar hologram 65 b is patterned withsmaller rectangles in the directions from the center of the light guideplate 63 b to both the light source 61 and the light path changing unit69. Therefore, the color impression of output light can be uniformlymaintained, and the light intensity of output light can be uniformlydistributed over the entire surface of the light guide plate 63.

[0113] Alternatively, the planar hologram 65 b can be patterned suchthat two types of rectangles have different diffraction efficiencydepending on the wavelength of light by having different grating depths.

[0114]FIG. 12A shows the pattern of a planar hologram 45 c having twodifferent periodical-shaped grating intervals formed on a light guideplate 43 c in a backlight with no light path changing units according tothe present invention. Referring to FIG. 12A, the planar hologram 45 cis formed on the entire surface of the light guide plate 43 c, such thatthe diffraction efficiency of light increases. This increases theprobability that light is emitted from the light guide plate 43 c. Inaddition, as described above, light having two different wavelengths canbe output at an angle of 80° or more to the plane of the light guideplate 43 c, such that the color impression of light reproduced on aliquid crystal panel is equalized, and that a uniform light intensitydistribution is obtained. This leads to the improvement of imagequality.

[0115] In a flat backlight having no light path changing units accordingto the present invention, the planar hologram 45 included on the lightguide plate 43 can be patterned with periodical stripes having no lessthan two different grating intervals. Each of the grating intervals canhave a width satisfying Equation 4, depending on the wavelength oflight.

[0116]FIG. 12B shows the pattern of a planar hologram 65 c formed on alight guide plate 63 c in the backlights according to the first throughfourth and sixth embodiments of the present invention. Referring to FIG.12B, the diffraction efficiency of a planar hologram 65 c becomes lowerin the directions toward the light source 61 and the light path changingunit 69. Alternatively, when the diffraction efficiency is uniform overthe surface of the light guide plate 63 c, the pattern of the planarhologram 65 c becomes smaller in the directions toward the light source61 and the light path-changing unit 69. Each of the grating intervals isformed depending on the wavelength of light as shown in FIG. 12A, whilethe pattern of the planar hologram 65 c has different densities over thesurface of the light guide plate 63 c. However, this configuration isessentially made to evenly distribute light intensity of output lightover the entire surface of the light guide plate 63 c, similar to theprinciple of patterning of the planar holograms used in the flatbacklight having no light path changing units according to the presentinvention.

[0117] Referring to FIG. 13, in order to form a planar hologram 45 on alight guide plate 43, first, the light guide plate 43 is coated with aphotosensitive material 50. Two incident light beams 31 a and 31 boutput from an identical light source are incident at anglesΘ_(1 l and Θ) ₂ upon the light guide plate 43 coated with thephotoresist 50, and interfere with each other. The interference betweenthe two incident light beams 31 a and 31 b produces an interferencepattern on the photoresist 50 of the light guide plate 43.

[0118] The incident light beams can be formed of a combination of aconvergent light beam and a parallel light beam or a combination of theconvergent light beam and a diffused light beam. The planar hologram 45,which is engraved according to the type of incident light beams, has agrating interval P according to Equation 5:

P=λ/(sinΘ₁+sinΘ₂)   (5)

[0119] wherein the grating interval P is set to be 2 μm or less.

[0120] The planar hologram 45 can be obtained by developing orchemically treating the interference pattern using a standard processfor the photoresist 50. The planar hologram 45 can be mass-producedthrough reproduction by stamping. Alternatively, the planar hologram 45can be formed by exposing the light guide plate 43 to light using amask.

[0121] In FIG. 13, if a volume hologram material such as dichromatedgelatin or silber halide is used as the photoresist, a volume hologrammay be obtained. However, the present invention refers to a planarhologram. A planar hologram is easily mass-produced using theabove-described method, and provides a high environmental reliability induplicates.

[0122] A planar hologram 45 having a uniform grating interval accordingto an embodiment of the present invention is manufactured by theinterference between two parallel light beams. In addition, a planarhologram 45 having a gradually varying grating internal can bemanufactured by changing the angle made by two light beams using aconvergent light beam or a parallel light beam.

[0123]FIG. 14 is a graph showing the diffraction efficiency ofP-polarized light and S-polarized light according to the depths of thegrating of a planar hologram when blue light with a 460 nm wavelengthand red light with a 620 nm wavelength are incident upon a planarhologram 45 with a grating interval of 440 nm. Here, the P-polarizedlight is indicated by transverse magnetic light (TM) and the S-polarizedlight is indicated by transverse electric light (TE). Accordingly,P-polarized light with a 460 nm wavelength is indicated by 460TM,S-polarized light with a 460 nm wavelength is indicated by 460TE,P-polarized light with a 620 nm wavelength is indicated by 620TM, andS-polarized light with a 620 nm wavelength is indicated by 620TE.

[0124] As shown in FIG. 14, the diffraction efficiency of light 620TEhas a maximum of about 20% at 0.3 μm and 0.6 μm grating depths, and thediffraction efficiency of light 620TM has a maximum of about 50% at a0.7 μm grating depth. It is known that each of the light 460TE and thelight 460TM has a diffraction efficiency of less than 10%.

[0125] When both a 620 nm light beam and a 460 nm light beam areincident upon the flat hologram 45 with a 440 nm grating interval and agrating depth of about 0.25 μmm, only polarized light TE is usuallyoutput because polarized light TE has a significantly high diffractionefficiency. Accordingly, specific polarized light can be strengthened.

[0126] A lighting system for emitting only specific polarized light withhigh efficiency increases the light intensity of output light byreducing a light loss. Hence, such a lighting system can be useful in adisplay device using polarization.

[0127] The effects of such a lighting system according to the presentinvention can be known from FIGS. 15A and 15B. FIG. 15A shows the lightintensity distribution of light transmitted by the light guide plate 43in the backlight with no light path changing unit according to thepresent invention. A light intensity of no less than 4,000 cd isdistributed along the horizontal axis 0°-180°. The light intensity atthe center of the light guide plate 43 is about 4,600 cd, and the otherareas of the light guide plate 43 have light intensity of no more than1,600 cd.

[0128]FIG. 15B shows the light intensity distribution of final lighttransmitted by a diffusion plate 49 through the light guide plate 43 inthe backlight of FIG. 7. Referring to FIG. 15B, the asymmetricalstructure of FIG. 15A is changed into a nearly symmetrical structure.The light intensity moves toward the vertical axis 90°-270° to form acircle around the center of the light guide plate 43. This leads touniform distribution of light. Here, the light intensity at the centerof the light guide plate 43 is about 2,400 cd.

[0129] It can be confirmed from FIGS. 15A and 15B that light can bevertically output by using a light guide plate and a diffusion platewithout using a prismatic plate.

[0130] In a backlight according to preferred embodiments of the presentinvention, a planar hologram formed on a light guide plate enables lightto be output at an angle nearly perpendicular to the light guide plate.That is, as the light efficiency can be increased just by using a planarhologram, a backlight according to the present invention can bemanufactured in a simple structure requiring no prismatic plates.

[0131] Flat backlights according to preferred embodiments of the presentinvention provide a maximum diffraction efficiency by making the most ofthe incident light with a high light intensity. In addition, the flatbacklights according to the present invention increase luminance andreduce light loss by uniformly distributing light intensity over theentire surface of a light guide plate. Therefore, backlights of goodperformances are provided.

[0132] As described above, a flat backlight according to the presentinvention can reduce a light loss and increase the light intensity ofoutput light by adopting a light path changing unit to maximallydiffract incident light having a maximum light intensity.

[0133] Also, a flat backlight according to the present invention canemit light nearly vertically by periodically patterning a planarhologram, such that it can be manufactured in a simple structureproviding a high luminous efficiency.

[0134] In addition, a flat backlight according to the present inventioncan increase the diffraction efficiency of specific polarized light bycontrolling the depth of grating formed on a planar hologram.

[0135] While this invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A backlight for a flat display device comprising:a light source; a light guide plate installed at one side of the lightsource, in which light from the light source travels while being totallyreflected; and a planar hologram formed on at least one surface of thelight guide plate, the planar hologram having a pattern of apredetermined grating interval and a predetermined grating depth inorder to diffract light at a predetermined angle toward the light guideplate.
 2. The backlight for a flat display device of claim 1, furthercomprising a light path changing unit installed at one side of the lightguide plate, the light path changing unit changing the path of lighttraveling in the light guide plate to make light incident upon theplanar hologram at an angle near the angle providing the maximumdiffraction efficiency.
 3. The backlight for a flat display device ofclaim 1, further comprising a reflecting plate installed on the rearside of the light guide plate, the reflecting plate reflecting lightdiffracted by the planar hologram and sending the diffracted light backto the light guide plate.
 4. The backlight for a flat display device ofclaim 2, further comprising a reflecting plate installed on the rearside of the light guide plate, the reflecting plate reflecting lightdiffracted by the planar hologram and sending the diffracted light backto the light guide plate.
 5. The backlight for a flat display device ofclaim 2 or 4, wherein the light path changing unit is a reflectivemirror that is located opposite to the light source and inclined at apredetermined angle.
 6. The backlight for a flat display device of claim2 or 4, wherein the light path changing unit is a reflective surface ofthe light guide plate, the reflective surface being located opposite tothe light source and inclined at a predetermined angle.
 7. The backlightfor a flat display device of claim 2 or 4, wherein the light pathchanging unit is a refracting element installed between the light sourceand the light guide plate or installed opposite to the side of the lightguide plate where the light source is installed.
 8. The backlight for aflat display device of claim 7, wherein the refracting element is arefractive lens.
 9. The backlight for a flat display device of claim 7,wherein the refracting element is a refractive grating.
 10. Thebacklight for a flat display device of claim 1 or 3, wherein thediffraction efficiency of the planar hologram becomes lower toward thelight source.
 11. The backlight for a flat display device of claim 1 or3, wherein the pattern of the planar hologram becomes smaller toward thelight source.
 12. The backlight for a flat display device of claim 2 or4, wherein the diffraction efficiency of the planar hologram becomeslower toward the light path changing unit.
 13. The backlight for a flatdisplay device of claim 2 or 4, wherein the pattern of the planarhologram becomes smaller toward the light path changing unit.
 14. Thebacklight for a flat display device of claim 9 or 11, wherein thegrating interval of the planar hologram is 2 μm or less.
 15. Thebacklight for a flat display device of claim 12, wherein the gratinginterval of the planar hologram is no greater than 2 μm.
 16. Thebacklight for a flat display device of claim 13, wherein the gratinginterval of the planar hologram is no greater than 2 μm.
 17. Thebacklight for a flat display device of claim 10 or 11, wherein thegrating interval of the planar hologram is composed of at least twotypes of grating intervals depending on the wavelength of light.
 18. Thebacklight for a flat display device of claim 12, wherein the gratinginterval of the planar hologram is composed of at least two types ofgrating intervals depending on the wavelength of light.
 19. Thebacklight for a flat display device of claim 13, wherein the gratinginterval of the planar hologram is composed of at least two typesdepending on the wavelength of light.
 20. The backlight for a flatdisplay device of any of claims 1 through 4, wherein the grating depthof the planar hologram is set so that the diffraction efficiency ratioof two polarized light beams that meet each other at a right angle is noless than 1.5.
 21. The backlight for a flat display device of any ofclaims 1 through 4, further comprising a diffusion plate installed onthe entire surface of the light guide plate to diffuse light emittedfrom the light guide plate.